This invention relates to a surface acoustic wave device of the type utilizing a piezoelectric substrate which comprises a cyrstalline zinc oxide coating layer on a base layer of an electrically insulating material.
As is well known, a surface acoustic wave device is obtained by providing interdigital electrodes onto a piezoelectric substrate. In this device the dimensions of the electrodes are made proportional to the wavelength .lambda. of surface acoustic waves to be propagated, and the wavelength is given by the equation .lambda.=.upsilon./f, where .upsilon. is the velocity of propagation of the waves (phase velocity) and f is the frequency. It becomes necessary, therefore, to extremely diminish the dimensions of the electrodes where the device is to be used at high frequencies such as VHF or UHF bands, and this offers difficulties to the manufacture of the electrodes. From this reason it is desirable to utilize a piezoelectric substrate in which the wave propagation velocity is as high as possible for a surface acoustic wave device for use at high frequencies.
A typical example of conventional piezoelectric substrate for surface acoustic wave devices is a double-layer substrate consisting of a base plate of fused quartz, or a different type of glass, and a surface region or coating layer of crystalline zinc oxide which is a piezoelectric material. However, this piezoelectric substrate is unsatisfactory for use at high frequencies because the phase velocity in this substrate is not so high, and, besides, the effective electromechanical coupling coefficient K.sup.2, a primary factor determining the characteristics of a surface acoustic wave device, of a device utilizing this substrate cannot be made sufficiently large. For example, the phase velocity is about 3300 m/s and the coefficient K.sup.2 is about 1.0% in the case of a substrate having a fused quartz plate and a zinc oxide layer coated on the glass plate.